HK-47 for sure! He could mock people and I'd mess with him by inserting the Protocol Pacifist Package

HK-47: "Answer: Simply that the distinction between 'killer' and 'killee' be a clear one. I cannot kill of my own volition, naturally."
Revan: "I don't think 'killee' is a word."
HK-47: "Expletive: Damn it, master, I am an assassination droid... not a dictionary!"

"Angry Statement: If I see one more meatbag attempt to attack a Jedi with a blaster pistol, I'll kill him myself!"
―HK-47

"They'll hunt me; They'll condemn me; Set the dogs on me; Because that's what needs to happen. Because sometimes, the truth isn't good enough. Sometimes, people deserve more. Sometimes, people deserve to have their faith rewarded. Not because they want to, not because of chance, but because they deserve to."

I choose the Juggernaut War Droid (Duwani Mechanical Products) for its effective use of outrigger jets (tansformable into aquatic propellers) and its main weapons: Left Arm - Wide-Beam Sonic Stunner, Right Arm - Shatter beam, as well as it's programming to carry Republic Pulse-Wave rifles, which could be modified to carry newer, better blaster rifle & blaster carbine weapons. As for combat effectiveness, look up the Great Droid Revolution (4015 B.B.Y.) Juggernauts shredded a platoon of rocket-jumpers in air over Monument Plaza.